By GUY GUGLIOTTA


ALTADENA, California - When space engineers made a map of a planet or a distant moon back in the old days, they made a picture of part of the surface, identified a couple of landmarks and measured the distance between them with a straightedge and a crayon.

“Then you punched the number into the computer and figured out the latitude and longitude,”recalled Robert W.Gaskell, a planet-mapping expert. The result was two-dimensional, with only hints of surface texture or complexity.

No more. With modern computers, digital photography and laser range-finding, scientists today can pinpoint the locations of their spacecraft, picking landing sites and otherwise describing the solar system’s extraterrestrial landscapes.

And nobody does it better than the 63-year-old Dr.Gaskell. With software he developed over a quarter-century , he can process hundreds of images in a few hours, slap them atop one another electronically like coats of paint and produce a topographical map so detailed that you often need a pair of 3-D glasses to appreciate what he has done.

He retired from the NASA Jet Propulsion Laboratory in 2006 so he could make maps full time, and he now works in a computer-filled office at his home in this suburb northeast of Los Angeles.

Dr.Gaskell is now mapping all of Mercury and eight moons of Saturn. He also has a NASA contract to do part of a topographical map of Earth’s Moon, and he is working to refine his model of the near- Earth asteroid Eros. He has made 12,000 overlay“maplets”of Eros from images produced by the NEAR spacecraft at the turn of the century.

In all, NEAR made 160,000 digital images, and“I’m doing 1,000 at a time,”Dr.Gaskell said.

Dr.Gaskell calls what he does “stereophotoclinometry.”Ideally he needs at least three images of the target landscape, usually taken by an orbiting spacecraft or a probe on a flyby to another destination. Only in rare cases can telescope images provide enough detail.

The sun angle must be different for each exposure so each image shows different shadows. By comparing the shadows, the software calculates slopes, which yield the altitudes of target features. The computer solves the equation in three dimensions, producing a patchlike topographical maplet.

The goal is to create a 3-D model of an entire body. A first take can be done quickly, but more images and more maplets produce more overlap, sharpening the topography.

It was in 1989 that Dr.Gaskell got an inkling of what might be possible, using images from the Voyager spacecraft to study Jupiter’s volcanic moon Io. He built his map by comparing the brightness of a feature in different images.“I intuitively knew that different illuminations could give more information,”he said.

“His approach is unique,”said the geophysicist Olivier Barnouin-Jha of the Johns Hopkins University Applied Physics Laboratory, a Gaskell collaborator.“But it’s not quite as nice as a black box. There’s a lot of handholding and agonizing that goes into this.”